Tourmaline is pyroelectric. A change in temperature generates an electrical charge across the crystal, which is why the stone has been known since antiquity for attracting ash and dust, and it is the only material in the ten-piece anniversary Royal Oak gem set that behaves this way. On a watch where the dial, the bezel and the entire bracelet are set in tourmaline, that is a practical characteristic rather than a curiosity: the surface will pick up fine dust more readily than sapphire or garnet would, particularly after a change in ambient temperature. Tourmaline is also the most chemically variable species in the set. The crystal accepts an unusually wide range of elements, which is why it occurs in more colours than any other gem material and why a single crystal can change colour along its own length, and selecting a run of pink baguettes that agree closely enough to set beside one another means rejecting a great deal of otherwise sound material.
The case is 37 mm across and 10 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut pink tourmalines. The plain 37 mm selfwinding Royal Oak of the same generation runs to 9.1 mm, so the complete setting adds nine tenths of a millimetre, against one and one tenth on the 41 mm reference, because smaller stones sit in shallower channels. The eight hexagonal screw positions remain and interrupt the continuous run of stones at fixed intervals. The dial is white gold set with baguette-cut pink tourmalines across its whole surface, with no applied hour-markers. The stones are the dial. The white gold Royal Oak hands are the only element above them. The bracelet carries the same setting to the clasp, tapering from the case and closing on an AP folding clasp. Pink against white gold gives the softest tonal relationship in the set, since the two are close enough in lightness that the boundary reads as a change of material rather than a change of colour, and at 37 mm across smaller stones that unity goes further still. The watch reads as one object at conversational distance and resolves into stones only close up, which is the opposite of what the ruby and spessartite versions do.
The function set is hours, minutes and centre seconds. There is no date. An aperture would break a continuous stone surface at the point where the run most needs to stay unbroken, and on the smaller dial the proportional damage would be greater. Centre seconds keeps all three hands on a single axis and avoids a second circular interruption. Time is read from hand position against the case, since the dial carries no markers. This is the least favourable legibility pairing in the set alongside blue topaz, because a pale stone under white gold hands offers almost no contrast and the hands are found by shape and length rather than by tone. That is inherent to choosing the lightest end of the range, and the alternative would have been to darken the hands and break the material logic of the watch.
The Royal Oak is the collection Audemars Piguet is built on, and the 37 mm anniversary references step through three degrees of setting. The 15550 is plain at 9.1 mm, the 15551 carries a diamond-set bezel over an unset dial at the same height, and this reference sets dial, bezel and bracelet and reaches 10 mm. Ten stones were used: pink tourmaline, ruby, blue sapphire, emerald, tsavorite, tanzanite, blue topaz, amethyst, yellow chrysoberyl and orange spessartite. Each was issued at 37 mm here and again at 41 mm at reference 15514, producing twenty references on one construction across two sizes. Blue is represented three times and green twice; pink, red, purple, yellow and orange appear once each, so this reference is the only route to its colour in either size. Neither of the two non-anniversary 41 mm graded dials uses tourmaline either, which makes the two single-stone references the whole of the pink in the programme.
This reference suits a collector who wants a gem-set Royal Oak that does not announce the gems. Pink tourmaline against white gold is the quietest combination available, and the 37 mm case reinforces that rather than working against it, since the smaller stones read as a continuous pale surface rather than as individually identifiable baguettes. That is a genuinely different proposition from the rest of the set, most of which is built to be noticed. It appeals to someone who wants the construction and the setting work without the colour statement, and to someone who prefers a stone that is chosen rather than recognised, since tourmaline carries less immediate name recognition than the corundums and beryls beside it. The legibility trade and the dust behaviour both have to be accepted as part of that choice. The anniversary designation ties production to the occasion rather than to an open catalogue run.
